Impedance computation for ablation therapy

ABSTRACT

This disclosure describes impedance computation techniques that may reduce the effect of crosstalk, thus generating more accurate impedance measurements. In particular, an ablation system models the electrical interaction among the active electrodes and a common return electrode using a star-configuration resistor model. The ablation system computes one or more parameters of the star-configuration resistor model and adjusts the therapy based on at least the computed parameters of the star-configuration resistor model.

This application claims the benefit of U.S. Provisional Application No.60/891,100, filed Feb. 22, 2007, the entire content of each of which isincorporated herein by reference.

TECHNICAL FIELD

The invention relates to medical devices and, more particularly, tomonitoring electrical impedances within a therapy delivery device.

BACKGROUND

Ablation therapy is a surgical technique used to destroy or selectivelyremove tissue to reduce or eliminate patient conditions. Many differenttechnologies are directed to tissue ablation. Such technologies includeelectrical ablation, ultrasound ablation, microwave ablation,cryoablation, conductive heating, radioactive heating, and combinationablation that includes more than one technology. Electrical ablationutilizes electrical current to heat the target tissue, and may includetechniques such as radio frequency (RF) ablation and other electricalablation technologies.

One example of an ablation therapy is treatment for benign prostatichyperplasia (BPH). BPH is a condition caused by the second period ofcontinued prostate gland growth. This growth begins after a man isapproximately 25 years old and may begin to cause health problems after40 years of age. The prostate growth eventually begins to constrict theurethra and may cause problems with urination and bladder functionality.Ablation therapy attempts to create a lesion in the prostate anddecrease the prostate size, restoring appropriate bladder function.

SUMMARY

In general, this disclosure describes techniques for modeling anelectrical interaction among active electrodes that delivery energy totissue of a patient and a common return electrode using astar-configuration resistor model. The star-configuration resistor modelis a model in which one terminal of each of the resistors is coupled ata central node. Processor may monitor and/or control a therapy beingdelivered to the patient based on one or more parameters of thestar-configuration model. As an example, processor may adjust one ormore parameters of the therapy based on impedances of thestar-configuration model computed using a series of energy bursts.

In one embodiment, a method for delivering therapy to a patientcomprises modeling an electrical interaction between a first and secondactive electrode that deliver energy to a target tissue location of apatient and a return electrode using a star-configuration resistor modelin which all of the resistors are coupled to each other at a commonnode, applying energy to one or more different combinations of the firstand second active electrodes to compute one or more parameters of thestar-configuration resistor model, delivering a therapy to the patient,and adjusting one or more parameters of the therapy based on at leastone of the computed parameters of the star-configuration resistor model.

In another embodiment, a therapy delivery device comprises a firstactive electrode that delivers energy to a target tissue location of apatient, a second active electrode that delivers energy to the targettissue location of the patient, a return electrode that operates as adestination for the energy delivered via the first and second activeelectrodes, a generator that delivers therapy energy to the patient viathe first and second active electrodes, and a processor that models anelectrical interaction between the first and second active electrode andthe return electrode using a star-configuration resistor model in whichall of the resistors are coupled to each other at a common node. Theprocessor causes the generator to apply energy to one or more differentcombinations of the first and second active electrodes, computes one ormore parameters of the star-configuration resistor model based onmeasurements made during application of energy to the differentcombinations of the active electrodes, and adjusts one or moreparameters of the therapy based on at least one of the computedparameters of the star-configuration resistor model.

In a further embodiment, a computer-readable medium comprisesinstructions that cause a processor to model an electrical interactionbetween a first and second active electrode that deliver energy to atarget tissue location of a patient and a return electrode using astar-configuration resistor model in which all of the resistors arecoupled to each other at a common node, apply energy to one or moredifferent combinations of the first and second active electrodes tocompute one or more parameters of the star-configuration resistor model,deliver a therapy to the patient, and adjust one or more parameters ofthe therapy based on at least one of the computed parameters of thestar-configuration resistor model.

The details of one or more embodiments of this disclosure are set forthin the accompanying drawings and the description below. Other features,objects, and advantages of this disclosure will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example ablation systemin conjunction with a patient.

FIGS. 2A and 2B are cross-sectional side views of a distal end of acatheter of an ablation device.

FIG. 3 is a cross-sectional front view of a distal end of a catheter inconjunction with a target tissue location.

FIG. 4 is a circuit diagram illustrating an example star-configurationresistor model that models the interaction among a first and secondactive electrode and a return electrode.

FIG. 5 is functional block diagram illustrating components of anexemplary therapy delivery device.

FIG. 6 is a flow diagram illustrating exemplary operation of an ablationsystem computing more accurate impedance measurements and controllingtherapy based on the impedance measurements in accordance with thetechniques of this disclosure.

FIG. 7 is a flow diagram illustrating exemplary operation of an ablationsystem adjusting one or more parameters of the delivered therapy basedon a comparison of the common impedance measured at two differentinstances in time.

FIG. 8 is a flow diagram illustrating exemplary operation of an ablationsystem adjusting one or more parameters of the delivered therapy basedon a comparison of the true impedances between the electrodes and areturn electrode.

DETAILED DESCRIPTION

This disclosure describes techniques for reducing the effects ofcrosstalk in electrical impedance measurements in a multi-electrodeablation system. During electrical ablation, such as radio frequency(RF) ablation, an ablation system delivers energy to two or moreelectrodes in contact with or in close proximity to a target tissuelocation within the body of a patient. The energy passes from theelectrodes of the ablation system to the target tissue location, causingheating of the tissue. The ablation system continues to deliver energyto the tissue location via the electrodes until the tissue location isheated to a high enough temperature to cause ablation of the tissue. Theablation system may deliver the energy to the tissue location via bothelectrodes simultaneously for a particular duration of time.

The amount of energy delivered to the target tissue location may beadjusted to control the ablation therapy. In some embodiments, theamount of energy delivered to each of the respective electrodes may beindependently controlled. To assist in controlling the energy deliveryof the ablation system, the impedance levels between each of theelectrodes to which energy is delivered and a common return electrodemay be monitored. One or more parameters of the therapy may becontrolled based on the respective impedance measurements. In thismanner, the ablation system may be configured to operate within specificimpedance limits to avoid damage to the target tissue location. In someablation systems, the electrodes are located in close proximity of eachother. This may result in crosstalk between the two or more electrodes,which may affect the impedance measurements between each of theelectrodes to which energy is delivered and the common return electrode.In some cases, crosstalk can result in erroneously high impedancemeasurements that result in erroneous reductions in the amount of energydelivered to the electrode.

The impedance measurement techniques described herein model theelectrical interaction among the active electrodes and a common returnelectrode using a star-configuration resistor model. Thestar-configuration resistor model is a model in which one terminal ofeach of the resistors is coupled at a central node. In fact, theresistors are only interconnected via this central node. The ablationsystem computes one or more parameters of the star-configurationresistor model and adjusts the therapy based on at least the computedparameters of the star-configuration resistor model. The parameters ofthe star-configuration model may be, for example, one or more impedancesalong the different legs of the star-configuration resistor model,impedances between each of the active electrodes and the common returnelectrode, voltages measured during a series of pulses, currentsmeasured during the series of pulses or the like. In one embodiment, theablation system may use the values of the parameters of thestar-configuration model to compute adjusted impedance values thatremove the crosstalk between the electrodes. Moreover, the ablationsystem may compute these more accurate impedance measurements withoutrequiring deactivation of one of the active electrodes during theablation therapy.

FIG. 1 is a conceptual diagram illustrating an example ablation system10 in conjunction with a patient 12. Ablation system 10 delivers therapyto treat a condition of patient 12. In particular, ablation system 10provides energy, such as RF energy, to a target tissue location 14 ofpatient 12 to heat the tissue in order to ablate at least a portion oftarget tissue location 14. In the example illustrated in FIG. 1, targettissue location 14 is tissue of the prostate gland. Ablation of prostatetissue may destroy a portion of the enlarged prostate caused by, forexample, benign prostatic hyperplasia (BPH). Ablation system 10 may alsobe used on target tissue locations other than the prostate. For example,ablation system 10 may provide ablation therapy to cardiac tissue,ovarian cysts, colon polyps, tumors, or any other internal or externaltissue of patient 12.

Ablation system 10 includes a therapy delivery device 15 coupled to anablation device 16. Therapy delivery device 15 includes a generator 17that generates energy to provide to target tissue location 14 forablation therapy. To this end, generator 17 may include circuitry fordeveloping RF energy from a direct current (DC) power source, such as arechargeable battery, or drawn from an alternating current (AC) powersource, such as an electrical outlet. Generator 17 may generate theenergy within defined therapy parameters to provide appropriate heatingof target tissue location 14. In some embodiments, generator 17 may be adifferent type of energy source. For example, generator 17 may convertpower from a power source to produce steam, mechanical energy, or anyother type of output that may perform ablation on patient 12. Otherenergy may be laser energy or ultrasound energy. In this manner,generator 17 may produce electrical, chemical, or mechanical energy.

Therapy delivery device 15 also includes a processor 18 that controlsgenerator 17 to deliver energy according to therapy parameters. Thetherapy parameters may include an amount of time the therapy is applied,voltage levels of the therapy and the like. As will be described in moredetail below, processor 18 may compute impedance measurements duringdelivery of ablation therapy in accordance with the techniques describedherein. Processor 18 may also control the delivery of the therapy totarget tissue location 14 based on the computed impedance measurements.Alternatively, processor 18 may display the computed impedancemeasurements to a clinician or other user of ablation system 10. In thiscase, the user may control the delivery of energy to target tissuelocation 14 based on the computed impedance measurements. Processor 18may comprise any one or more of a microprocessor, digital signalprocessor (DSP), application specific integrated circuit (ASIC),field-programmable gate array (FPGA), or other digital logic circuitry.

Therapy delivery device 15 transmits the energy through an electricalcable 20 to ablation device 16. Electrical cable 20 may include one ormore conductors that electrically couple generator 17 to ablation device16 to provide the energy to ablation device 16. Alternatively, generator17 and ablation device 16 may be coupled via more than one electricalcable. Ablation device 16 transmits the energy through a catheter 22 anddelivers the energy to target tissue location 14 via electrodes 24A and24B (collectively, “electrodes 24”). In the example illustrated in FIG.1, electrodes 24 comprise needle electrodes that extend from catheter 22to penetrate target tissue location 14. However, other types ofelectrodes may be used in place of needle electrodes. For example,ablation device may deliver energy to target tissue location 14 via ringelectrodes located on the distal end of catheter 22.

Catheter 22 includes one or more channels that house needle electrodes24. In some embodiments, each of needle electrodes 24 is located withina separate channel. Catheter 22 may include additional channels forother features of ablation device 16, such as a channel for insertion ofcystoscope or a channel for a fluid delivery tube. Thus, catheter 22 mayprovide a conduit for both energy from generator 17 and fluid from apump (not shown in FIG. 1). Catheter 22 may be formed in variousdiameters to enter through different orifices of patient 12. Catheter 22may be produced in different lengths with different configurations ofelectrodes based on the type of therapy to be delivered or the locationof patient 12 to which the therapy will be delivered.

Catheter 22 includes shaft 28 and tip 30. Shaft 28 may be permanentlycoupled to a housing of catheter 22. Alternatively, shaft 28 may bedetachable and simply locked in place for a treatment session. In thismanner, shaft 28 may be replaced and discarded after a single use. Shaft28 may couple to tip 30 by, for example, a screw mechanism, snap fit, oradhesives. Tip 30 includes one or more openings that allow theelectrodes 24 to exit catheter 22 and extend into target tissue location14. Shaft 28 may be a rigid structure that is manufactured of stainlesssteel or other metal alloy and insulated with a polymer such as nylon orpolyurethane. Alternatively, shaft 28 may be constructed of a rigidpolymer or composite material. In some applications, shaft 28 may bemade of a slightly more flexible material. In some embodiments, tip 30may be constructed of an optically clear polymer such that the clinicianmay view the urethra or other bodily cavity using a cystoscope duringinsertion of catheter 22 into patient 12. However, a transparent tip 30is not required.

Ablation system 10 further includes a return electrode 26 that operatesas a destination for the energy delivered via electrodes 24. Returnelectrode 26 may, for example, be a ground pad placed at an exteriorlocation on patient 12 on or near the opposing side of target tissuelocation 14. In the example illustrated in FIG. 1, return electrode 26is placed on the lower back of patient 12. Alternatively, returnelectrode 26 may extend from ablation device 16 to be placed within thepatient. In either case, energy delivered by generator 17 travels fromrespective ones of electrodes 24 to return electrode 26.

In some embodiments, generator 17 may independently control the amountsof energy delivered to each of electrodes 24. Thus, generator 17 maydeliver different amounts of energy to each of the electrodes 24. Morespecifically, generator 17 may deliver a first amount of energy toelectrode 24A via a first conductor within electrical cable 20 anddeliver a second amount of energy to electrode 24B via a secondconductor within electrical cable 20. Alternatively, in the case inwhich more than one electrical cable couples generator 17 to ablationdevice 16, generator 17 may deliver a first amount of energy toelectrode 24A via a first electrical cable and may deliver a secondamount of energy to electrode 24B via a second electrical cable. In thismanner, ablation system 10 operates each of electrodes 24 in anindependent, monopolar fashion.

When the energy is being delivered, target tissue location 14 mayincrease in temperature, thus destroying a certain volume of tissue.This heating may last a few seconds or a few minutes, depending on thecondition of target tissue location 14 and the desired therapy. Tocontrol and/or monitor the ablation therapy, ablation system 10 measuresimpedances between each of electrodes 24 and common return electrode 26.Based on the respective impedance measurements, the one or moreparameters of the therapy being delivered via electrodes 24 may beadjusted. In one embodiment, processor 18 adjusts one or more parametersof the delivered therapy when one of the impedance measurements exceedsa threshold impedance. For example, processor 18 may reduce the amountof energy delivered to the one of electrodes 24 corresponding to thehigh impedance measurement. In this manner, ablation system 10 operateswithin configured impedance limits. Alternatively, processor 18 maydisplay the computed impedance measurements to a clinician or other userof ablation system 10. In this case, the user may control the deliveryof energy to target tissue location 14 based on the computed impedancemeasurements.

Since electrodes 24 are located in close proximity to one another whendisposed in target tissue location 14, crosstalk between electrodes 24may affect the impedance measurements. More specifically, current maytravel from one of electrodes 24 down the circuit of the other one ofelectrodes 24 to generator 17. This crosstalk can result in erroneousimpedance measurements. In some cases, the result is an erroneously highimpedance measurement. The erroneously high impedance measurement mayresult in unwanted reductions in energy to the one of electrodes 24corresponding with the high impedance measurement. Computing impedancemeasurements in accordance with the star-configuration resistor modelmay reduce the effect of crosstalk on the impedance measurements, thusproviding more accurate impedance measurements. In particular, processor18 models the electrical interaction among electrodes 24 and commonreturn electrode 26 using a star-configuration resistor model, andcomputes the impedance measurements as a function of this model.

Processor 18 computes one or more parameters of the star-configurationmodel via a series of energy bursts on electrodes 24. In particular,ablation system 10 measures one or more voltages and currents duringdelivery of the series of energy bursts to different electrodecombinations, and computes one or more impedances based on the measuredvoltages and currents. The measured voltages, the measured currents, andthe computed impedances comprise the one or more parameters of thestar-configuration resistor model. In one embodiment, processor 18 maycompute a common impedance to return electrode 26. Subsequently, duringthe ablation therapy, processor 18 measures a number of electricalparameters of the therapy, such as impedances of electrodes 24, currentsrunning through electrodes 24, and the like. Using the electricalparameters measured during the ablation therapy as well as one or moreof the computed parameters of the star-configuration model, such as thevoltages, currents or computed impedances of the model, processor 18calculates adjusted impedances between electrodes 24 and returnelectrode 26. Again, the electrical parameters are those parametersmeasured during the therapy while the parameters of thestar-configuration resistor model are either computed prior to beginningthe therapy or during an interruption in the therapy.

Processor 18 may compute the impedances of the star-configurationresistor model prior to beginning delivery of the therapy to thepatient. Thus, the impedance measurement techniques described herein maybe performed on-the-fly without requiring periodic deactivation ofelectrodes 24 during the ablation therapy. Therefore, there is nodisruption to the ablation therapy. Alternatively, processor 18 maycompute the impedances of the star-configuration resistor model afterbeginning delivery of the therapy to the patient. In this case,processor 18 may interrupt delivery of the therapy to compute theimpedances of the star-configuration resistor model.

In another embodiment, processor 18 may compute a common impedance attwo different instances of time. For example processor 18 may computethe common impedance to return electrode 26 prior to beginning therapyand then after beginning therapy, or at two different instances in timeafter beginning delivery of the therapy. Processor 18 may compare thecomputed common impedances to return electrode 26 and adjust one or moreparameters of the therapy based on the comparison. For example, if thedifference between the two computed common impedances exceeds athreshold, it may be an indication that placement of the electrodes isnot in an ideal location or that one of electrodes 24 is broken. Thus,the user of ablation system 10 may adjust the location of electrodes 24or remove electrodes 24 entirely and test them outside of the patient. Asimilar determination may be made using parameters of the starconfiguration model prior to initiating therapy.

Ablation device 16 may be embodied as a hand-held device as shown inFIG. 1. Ablation device 16 may include a trigger 32 to control the startand stop of therapy. Trigger 32 may be pressure sensitive, whereincreased pressure of trigger 32 provides an increased amount of energyto the tissue of target tissue location 14. Trigger 32 may also be usedto deploy electrodes 24 into target tissue location 14.

In some cases, ablation device 16 may only be used for one therapysession. Reuse may cause infection and contamination, so it may bedesirable for ablation device 16 to only be used once per patient.Alternatively, ablation device 16 may be used more than once, butcatheter 22 may be discarded after each use, and replaced with a newcatheter for the next therapy session.

Although the techniques of this disclosure are described in terms of“dry” ablation therapy, the techniques are equally applicable in anablation system 10 that delivers a conductive fluid to interact with theenergy being delivered by electrodes 24, often referred to as “wet”ablation. Delivering a conductive fluid for wet ablation may increasethe effective heating area of electrodes 24, thus increasing therapyefficacy. In a wet ablation system, therapy delivery device 15 mayinclude a pump (not shown in FIG. 1) to pump fluid through a tube (notshown in FIG. 1) into ablation device 16. The fluid is then deliveredthrough catheter 22 to target tissue location 14. In some embodiments,the conductive fluid may exit small holes in needle electrodes 24 andflow around electrodes 24. The conductive fluid, e.g., saline, mayincrease the effective heating area and decrease the heating time foreffective treatment.

FIGS. 2A and 2B are cross-sectional side views of a distal end of acatheter, such as catheter 22 of ablation device 16 of FIG. 1. As shownin FIG. 2A, shaft 28 is coupled to tip 30 at the distal end of catheter22. Tip 30 includes protrusion 38 that aids the insertion of catheter 22through the urethra or other orifice of patient 12.

Catheter 22 includes a channel 40A that houses needle electrode 24A andallows needle electrode 24A to exit tip 30. Channel 40A extends from tip30 through shaft 28 into the housing of ablation device 16 (FIG. 1). Thecurved portion of channel 40A in tip 30 deflects needle electrode 24Asuch that needle electrode 24A penetrates target tissue location 14 froma side of catheter 22. The curvature of channel 40A may be altered toproduce different entry angles of needle electrode 24A. Thus, needleelectrode 24A may exit at or near the side of catheter 22, wherein theside is a lengthwise edge substantially facing the wall of target tissuelocation 14. In some embodiments, channel 40A may be configured suchthat needle electrode 24A exits from the distal end of catheter 22instead of from the side of catheter 22.

Needle electrode 24A is insulated with sheath 42A, such that the exposedportion of needle electrode 24A may deliver energy to target tissuelocation 14. Needle electrode 24A is deployed from tip 30 of catheter22. Needle electrode 24A may, for example, be deployed via theclinicians interaction with trigger 32 (FIG. 1) of ablation device 16.FIG. 2B illustrates the distal end of catheter 22 after needle electrode24A has been deployed from tip 30. The exposed length E of needleelectrode 24A is variable by controlling the position of sheath 42A.Exposed length E may be controlled by the clinician to be generallybetween 1 mm and 50 mm. Once needle electrode 24A is deployed, needleelectrode 24A may be locked into place until the ablation therapy iscompleted.

Ablation system 10 delivers energy to target tissue location 14 viadeployed needle electrode 24A as well as needle electrode 24B (not shownin FIGS. 2A and 2B). Needle electrode 24B resides behind needleelectrode 24A and thus cannot be seen in FIGS. 2A and 2B. Needleelectrode 24B may be substantially similar to needle electrode 24A. Asdescribed above, energy may be transferred between electrodes 24A and24B due to the close proximity of electrodes 24A and 24B within targettissue location 14. The cross talk may result in erroneous impedancemeasurements. The impedance measurement techniques described in thisdisclosure, however, may reduce the effect the crosstalk has on theimpedance measurements. Moreover, the impedance measurement techniquesmay allow for better control of the delivery parameters of the therapy,as described herein.

In some embodiments, needle electrode 24A does not deliver a conductivefluid to the target tissue. In this case, needle electrode 24A may besolid or hollow and act as a dry electrode. Delivering energy throughneedle electrode 24A without a conductive fluid may simplify theablation procedure and reduce the cost of ablation therapy.Alternatively, needle electrode 24A may be a hollow needle which allowsconductive fluid, e.g., saline, to flow from therapy delivery device 15to target tissue location 14. To this end, needle electrode 24A mayinclude multiple holes 43 which allow the conductive fluid to flow intotarget tissue location 14 and increase the effective are to which needleelectrode 24A may deliver therapy. The conductive fluid may also moreevenly distribute the RF energy to the tissue to create more uniformlesions.

FIG. 3 is a cross-sectional front view of an example a distal end of acatheter, such as catheter 22 of FIGS. 1 and 2, in conjunction withtarget tissue location 14. The example illustrated in FIG. 3 may, forexample, be a cross-sectional front view of tip 30 of FIG. 2B taken fromA to A′. As shown in FIG. 3, needle electrodes 24 are deployed from tip30 of catheter 22 into target tissue location 14. Needle electrode 24Ais housed within channel 40A and needle electrode 24B is housed withinchannel 40B. Channel 40B may mirror the path of channel 40A shown inFIGS. 2A and 2B. Channels 40A and 40B may or may not be identical indiameter.

In some embodiments, needle electrodes 24A and 24B are partially coveredby sheathes 42A and 42B, respectively. As described above, the exposedlength of needle electrodes 24 may be variable by controlling theposition of sheaths 42. Additionally, sheaths 42 may prevent fluids fromentering channels 40 when catheter 22 is used in a wet ablation system.Needle electrodes 24 may be constructed of similar materials ordifferent materials. Exemplary materials may include stainless steel,nitinol, copper, silver, or an alloy including multiple metals. In anycase, each of needle electrodes 24 may be flexible and conductelectricity to promote ablation therapy.

In the example illustrated in FIG. 3, needle electrodes 24 are deployedto the same extended length. Alternatively, needle electrodes 24 may bedeployed to different extended lengths. The angle at which needleelectrodes 24A and 24B exit tip 36 with respect to each other may bevaried based on the procedure for which catheter 22 is being used.Generally, the angle between needle electrodes 24A and 24B is between 0degrees and I₂₀ degrees.

As described above, energy is delivered from generator 17 (FIG. 1) totarget tissue location 14 via electrodes 24. The energy delivered toeach of electrodes 24 flows through target tissue location 14 and body48 of patient 12 to return electrode 26. Thus, return electrode 26operates as a common destination for the energy delivered via electrodes24. As illustrated in FIG. 3, return electrode 26 is placed on or nearthe opposing side of target tissue location 14. Arrow 50 illustrates theflow of a portion of the energy from electrode 24A to return electrode26. Arrow 52 illustrates the flow of a portion of the energy fromelectrode 24A to return electrode 26. A substantial portion of theenergy delivered to target tissue location 14 via electrodes 24 followthe path of arrows 50 and 52. However, since electrodes 24 are locatedin such close proximity to one another when disposed in target tissuelocation 14, some energy, e.g., in the form of current, may travel fromone of electrodes 24 to the other one of electrodes 24. In other words,the close proximity of electrodes 24 may result in crosstalk betweenelectrodes 24. Arrow 54 illustrates the crosstalk between electrodes 24.

Ablation system 10 may measure impedances between electrodes 24A andreturn electrode 26 and between electrode 24B and return electrode 26 tomonitor and control the ablation therapy. If, for example, the impedancebetween one of electrodes 24 and return electrode 26 rises to anundesirable level, ablation system 10 may adjust the amount of energybeing delivered to the respective one of electrodes 24. As an example,if the impedance between electrode 24A and return electrode 26 exceeds athreshold, processor 18 (FIG. 1) or a user may reduce the amount ofenergy delivered by generator 17 to electrode 24A. Due to the crosstalkbetween electrodes 24, however, the impedance measurements betweenelectrodes 24 and return electrode 26 may not be accurate. In fact,sometimes the crosstalk may result in erroneously high impedancemeasurements. In some cases the erroneously high impedance measurementsresult in inadvertent reductions in the amount of energy delivered toone or both of electrodes 24. To more accurately measure the impedancebetween electrodes 24 and the common return electrode 26, ablationsystem 10 models the electrical interaction among electrode 24A,electrode 24B, and common return electrode 26 using a star-configurationresistor model and computes impedance values based on the model.

FIG. 4 is a circuit diagram illustrating an example star-configurationresistor model that models the interaction among electrodes 24 andreturn electrode 26. In the example illustrated in FIG. 4, resistorsR₁₁, R₂₂ and R₃₃ are coupled to form the star-configuration resistormodel that models the electrical interaction among electrodes 24 andreturn electrode 26. The star-configuration resistor model is a model inwhich one terminal of each of resistors R₁₁, R₂₂ and R₃₃ is coupled at acentral node 66. In particular, R₁₁ is an impedance on the leg of starnetwork 60 from electrode 24A, R₂₂ is an impedance on the leg of starnetwork 60 from electrode 24B, and R₃₃ is a common impedance to returnelectrode 26. In other words, R₁₁ is the impedance from node 62 to node66, R₂₂ is the impedance from node 64 to node 66, and R₃₃ is theimpedance from node 66 to node 68. Node 62 and 64 refer to the exposedportions of needle electrodes 24A and 24B, respectively, within targettissue location 14. Node 66 may represent a location just outside of aprostate of patient 12. Node 68 represents a point of contact betweenthe body of patient 12 and return electrode 26, e.g., the small of theback in the case of treatment of a prostate condition.

Processor 18 uses model 60 to compute a more accurate approximation ofthe true impedance between each of electrodes 24 and return electrode26. The basic electrical formulae for star resistor network model 60are:

V ₁ =I ₁ *R ₁₁+(I ₁ +I ₂)*R ₃₃  (1)

V ₂ =I ₂ *R ₂₂+(I ₁ +I ₂)*R ₃₃,  (2)

where V₁ is a voltage on electrode 24A, I₁ is a current throughelectrode 24A, V₂ is a voltage on electrode 24B, and I₂ is a currentthrough electrode 24B. Voltages V₁ and V₂ and currents I₁ and I₂ may,for example, be measured by one or more sensors within generator 17.

The true impedance between each of electrodes 24 and return electrode 26is the impedance when the respective one of electrodes 24 is active. Forexample, the true impedance between electrode 24A and return electrode26 is the impedance measured when electrode 24A is active and electrode24B is inactive, i.e., no energy is transmitted via electrode 24B.Likewise, the true impedance between electrode 24B and return electrode26 is the impedance measured when electrode 24B is active and electrode24A is inactive. Thus, the true impedance is the impedance between theactive one of electrodes 24 and return electrode 26 without anycrosstalk. The true impedances between electrodes 24 and returnelectrode 26 can be defined as follows:

R ₁₃ =R ₁₁ +R ₃₃  (3)

R ₂₃ =R ₂₂ +R ₃₃,  (4)

where impedance R₁₃ is the electrical impedance between electrode 24Aand return electrode 26 and impedance R₂₃ is the electrical impedancebetween electrode 24B and return electrode 26. In terms of the nodes ofthe circuit diagram, impedance R₁₃ is the electrical impedance betweennode 62 and node 68 and impedance R₂₃ is the electrical impedancebetween 64 and 68. In accordance with the techniques of this disclosure,processor 18 computes one or more parameters of the star configurationmodel and adjusts delivery of the therapy as a function of one of thecomputed parameters of the star-configuration resistor model. Asdescribed below, the computed parameters may comprise impedance valuescomputed during a series of energy bursts.

When both electrodes 24A and 24B are active simultaneously, as is thecase during delivery of ablation therapy, the measured impedances withthe crosstalk are:

R ₁ =V ₁ /I ₁  (5)

R ₂ =V ₂ /I ₂  (6)

where R₁ is the measured impedance of electrode 24A and R₂ is themeasured impedance of electrode 24B. As described above, voltages V₁ andV₂ and currents I₁ and I₂ may be measured by one or more sensors withingenerator 17, and may include crosstalk, e.g., in the form of current,from the other one of electrodes 24.

In one example, processor 18 may use the parameters, e.g., computedimpedances, of the star-configuration model to compute a more accurateapproximation of the true impedances (R₁₃ and R₂₃), i.e., the impedancevalues without the crosstalk. In other words, processor 18 computes amore accurate impedance measurement that accounts for the crosstalkbetween each of electrodes 24. Mathematical manipulation of formulae(1)-(4) provides the following equations for computing adjustedimpedances between each of electrodes 24 and return electrode 26:

R ₁₃ =R ₁₁ +R ₃₃ =R ₁ −R ₃₃*(I ₂ /I ₁)  (7)

R ₂₃ =R ₂₂ +R ₃₃ =R ₂ −R ₃₃*(I ₁ /I ₂).  (8)

Therefore, it is possible to compute more accurate approximation of thetrue impedances (R₁₃ and R₂₃) during ablation therapy using the measuredimpedances of electrodes 24A and 24B (i.e., R₁ and R₂), electricalcurrent measurements through electrode 24A and 24B (i.e., I₁ and I₂),and the common impedance to electrode 26 (R₃₃). The variables R₁, R₂, I₁and I₂ may be measured during delivery of ablation therapy. In someembodiments, ablation system 10 includes one or more sensors thatmeasure R₁, R₂, I₁ and I₂ during delivery of ablation therapy, i.e.,while electrodes 24 are “active.” In this manner, R₁, R₂, I₁ and I₂ aredirectly measured with hardware instrumentation within ablation system10. In some embodiments, the impedance to common electrode 26 (R₃₃) maybe computed prior to the beginning of the ablation therapy withoutdeactivating one of electrodes 24. In some embodiments, processor 18 maytemporarily interrupt delivery of the therapy to compute R₃₃.Alternatively, processor 18 of ablation system 10 may pre-compute thecommon impedance to electrode 26 (i.e., R₃₃) prior to initiation of theablation therapy.

Processor 18 may compute R₃₃ using a series of energy bursts ondifferent combinations of electrodes 24. Processor 18 may compute R₃₃based on measurements, e.g., impedance measurements made during deliveryof the series of energy bursts. Generator 17 delivers an energy burst toelectrode 24A without delivering any energy to electrode 24B, i.e., afirst combination of electrodes. During delivery this energy burst toonly electrode 24A, ablation system 10 measures the impedance betweenelectrode 24A and common return electrode 26 (i.e. R₁₃). Generator 17also delivers an energy burst to electrode 24B without delivering anyenergy to electrode 24A, i.e., a second combination of electrodes.Again, ablation system 10 measures the impedance between electrode 24Band common return electrode 26 during delivery of the energy burst toonly electrode 24B (i.e. R₂₃). Generator 17 delivers energy bursts ofequal amplitude voltage, i.e., V₁=V₂ or I₁*R₁=I₂*R₂, to both electrode24A and 24B concurrently, i.e., a third combination of electrodes, andmeasures the impedance of both electrodes 24A and 24B (i.e., R₁ and R₂).Processor 18 uses the impedance measurements made during the series ofenergy bursts to compute R₃₃ according to the equation:

R ₃₃=((R ₁ −R ₁₃)/(R ₁ /R ₂)+(R ₂ −R ₂₃)/(R ₂ /R ₁))/2.  (9)

As described above, in some embodiments a physician, a clinician, acertified technician, or anyone trained to use ablation system 10 may,for example, deliver the initial energy bursts immediately prior to thebeginning of the ablation therapy, i.e., before both RF electrodes areactivated.

Processor 18 computes adjusted impedance measurements between each ofelectrodes 24 and common electrode 26 using impedances measured forelectrodes 24A and 24B (i.e., R₁ and R₂), currents measured throughelectrodes 24A and 24 B (i.e., I₁ and I₂), and the computed commonimpedance to return electrode 26 (R₃₃). More specifically, processor 18may compute the adjusted impedance measurements on-the-fly using thefollowing equations:

R ₁₃ =R ₁ −R ₃₃*(I ₂ /I ₁)  (10)

R ₂₃ =R ₂ −R ₃₃*(I ₁ /I ₂).  (11)

In this manner, processor 18 may compute more accurate impedancesbetween electrodes 24 and return electrode 26 (R₁₃ and R₂₃) on-the-fly,i.e., without disruption of the ablation therapy.

Processor 18 may use the adjusted impedance measurements to controldelivery of the energy to target tissue location 14. In particular, whenone of the impedance measurements exceeds a threshold limit, processor18 may cause generator 17 to reduce the amount of energy delivered tothe one of electrodes 24 corresponding to the high impedancemeasurement. Because the amount of energy delivered to each ofelectrodes 24 is controlled independently, generator 17 may continue todeliver the amount of energy initially being delivered to the other oneof the electrodes 24, i.e., the electrode not associated with the highimpedance measurement. In other words, only the energy delivered to oneof the electrodes need be adjusted. Alternatively, processor 18 maydisplay the impedance measurements to the user of ablation system 10,and the user may make manual adjustments to control the amount of energydelivered to electrodes 24 based at least in part on the impedancemeasurements.

In some embodiments, processor 18 may directly use the computedimpedances of the star configuration model to adjust one or moreparameters of the delivered therapy. For example, processor 18 maycompute the common impedance to return electrode 26 (R₃₃) at twodifferent instances of time and adjust the parameters of the therapybased on a comparison of the R₃₃ values at the different times. Forexample processor 18 may compute the common impedance to returnelectrode 26 prior to beginning therapy and then after beginningtherapy, or at two different instances in time after beginning deliveryof the therapy. Processor 18 may adjust the parameters of the therapy ifthe difference between the two computed common impedances exceeds athreshold. The user of ablation system 10 may adjust the location ofelectrodes 24 or remove electrodes 24 entirely and test them outside ofthe patient when the difference exceeds a threshold.

A similar determination may be made using impedances of the starconfiguration model prior to initiating therapy. For example, ablationsystem 10 may measure the impedance between electrode 24A and commonreturn electrode 26 (i.e. R₁₃) during delivery an energy burst toelectrode 24A without delivering any energy to electrode 24B. Ablationsystem 10 may also measures the impedance between electrode 24B andcommon return electrode 26 during delivery of an energy burst toelectrode 24B without delivering any energy to electrode 24A. Processor18 may then compare the measured impedances and adjust one or moreparameters of the therapy when the difference between the first andsecond measured impedances exceeds a threshold. In this case, it isdesirable that the energy burst applied to each of the electrodes 24 beof equal amplitude.

FIG. 5 is a functional block diagram illustrating components of anexemplary therapy delivery device, such as therapy delivery device 15 ofFIG. 1. Therapy delivery device 15 includes a generator 17, a processor18, a memory 70, a display 72, a connector block 74, a pump 78, acommunication interface 80, and a measurement circuit 86. As shown inFIG. 5, connector block 74 is coupled to cable 16 for delivering energyproduced by generator 17 and detecting tissue properties, e.g.,impedance, with measurement circuit 86. Pump 78 produces pressure todeliver fluid through tube 76.

Processor 18 controls generator 17 to deliver energy therapy throughconnector block 74 according to therapy parameter values stored inmemory 70. Processor 18 may receive such parameter values from display72 or communication interface 80, and store the parameter values inmemory 70. When signaled by the clinician to begin energy therapy (e.g.,ablation therapy), which may be a signal from therapy device 16 conveyedthrough connector block 74, processor 18 computes the common impedanceto return electrode 26 (FIG. 1) via a series of energy bursts deliveredby generator 17 through connector block 74 to different combinations ofelectrodes 24 (FIG. 1) in accordance with the techniques describedherein. After pre-computing the common impedance to return electrode 26,processor 18 communicates with generator 17 to produce the appropriateenergy. As described above, generator 17 may deliver energy to each ofelectrodes 24 (FIG. 1) independently. In other words, generator 17 mayindependently control the amount of energy delivered to each ofelectrodes 24, thus permitting delivery of different amounts of energyto each of electrodes 24.

Measurement circuit 86 may be configured to measure one or moreelectrical operating parameters of ablation delivery device 16. Forexample, measurement circuit 86 may measure impedances betweenelectrodes 24 and return electrode 26, a voltage on one or both ofelectrodes 24, a current through one or both of electrodes 24, or thelike. Processor 18 computes adjusted impedances between electrodes 24and return electrode 26 using the measured operating parameters and thepre-computed common impedance to return electrode 26. For example,processor may compute the adjusted impedances in accordance withequations (10) and (11).

Processor 18 monitors the computed impedances and may make modificationsto the energy delivery or fluid delivery based upon the measured tissueproperty. Processor 18 may, for example, compare the computed impedancesto a threshold set by the clinician or a predetermined program. Inresponse to identifying an impedance between one of electrodes 24 andreturn electrode 26 that exceeds the threshold, processor 18 may causegenerator 17 to increase or decrease the amount of energy delivered tothe respective one of electrodes 24.

In some embodiments, processor may additionally communicate with pump 78to provide fluid, as needed, to irrigate the ablation site or providefluid to the electrodes during wet electrode ablation. Processor 18 mayincrease or decrease the amount of fluid delivered via pump 78 based onthe computed impedances. Modifying the energy or fluid delivery may bedone individually or simultaneously.

Alternatively, processor 18 may display the computed impedances to auser of ablation system 10 via display 72. The user may then manuallymonitor and adjust the ablation therapy delivered to patient 12.Additionally, processor may detect high impedance measurements and issuean alert (either audible or visual) to the user via display 72.

Connector block 74 may contain an interface for a plurality ofconnections, not just the connection for cable 16. These otherconnections may include one for a return electrode or for separatetissue property sensors. As mentioned previously, connector block 74 maybe a variety of blocks used to diagnose or treat a variety of diseases.All connector blocks may be exchanged and connect to processor 18 forproper operation.

Processor 18 may also control data flow from the therapy. Data such asRF energy produced, tissue properties measured from measurement circuit86, and fluid flow may be channeled into memory 70 for analysis.Processor 18 may comprise any one or more of a microprocessor, DSP,ASIC, FPGA, or other digital logic circuitry. Memory 70 may includemultiple memories for storing a variety of data. For example, one memorymay contain therapy parameters, one may contain generator operationalfiles, and one may contain measured therapy data. Memory 70 may includeany one or more of a random access memory (RAM), read-only memory (ROM),electronically-erasable programmable ROM (EEPROM), flash memory, or thelike.

FIG. 6 is a flow diagram illustrating exemplary operation of ablationsystem 10 computing more accurate impedance measurements and controllingtherapy based on the impedance measurements in accordance with the oneof the techniques of this disclosure. Ablation system 10 delivers anenergy burst on electrode 24A (FIG. 1) and measures the impedancebetween electrode 24A and common return electrode 26, i.e., R₁₃ (82).The energy is delivered to electrode 24A without any energy beingdelivered to electrode 24B. Ablation system 10 also delivers an energyburst on electrode 24B and measures the impedance between electrode 24Band return electrode 26, i.e., R₂₃ (84). The energy is delivered toelectrode 24B without any energy being delivered to electrode 24A.Additionally, ablation system 10 delivers energy of equal amplitudevoltage on both electrode 24A and 24B, and measures impedance on bothelectrodes 24A and 24B (R₁ and R₂) (86). R₁ and R₂ may, for example, becomputed using equation (5) and (6) above, in which some of thevariables will be known and others may be measured. For example, V₁ andV₂ may be known and I₁ and I₂ may be measured upon delivery of theenergy to electrodes 24. Processor 18 computes a common impedance toreturn electrode 26 (i.e., R₃₃) using the variables measured during theseries of energy pulses (88). In particular, processor 18 may computeR₃₃ using equation (9) above. The series of energy pulses describedabove may be delivered in any order to obtain the variables forcomputing R₃₃. Moreover, the series of energy bursts may be deliveredprior to initiation of the therapy or after beginning the therapy. Inthe latter case, the therapy may be temporarily disrupted.

During delivery of the ablation therapy, ablation system 10 measuresimpedance values between each of electrodes 24 and the return electrode26 as well as current levels through each of electrodes 24 (90). Basedon these measurements, processor 18 computes more accurate impedancemeasurements between each of electrodes 24 and return electrode 26 toaccount for cross talk between electrodes 24 (92). Processor 18 may, forexample, compute the adjusted impedance values in accordance withequations (10) and (11). Moreover, processor 18 may compute these moreaccurate impedance values without disabling one of the electrodes 24 anddisrupting the ablation therapy.

Processor 18 determines whether either of the adjusted impedance valuesexceeds a threshold impedance limit (94). If neither of the adjustedimpedance values exceeds the threshold limit, ablation device continuesto periodically measure impedances and currents and compute adjustedimpedance values. If either of the adjusted impedance values does exceedthe threshold limit, however, generator 17 adjusts one or moreparameters of the delivered therapy (96). In one example, processor 18may cause generator 17 to reduce the amount of energy delivered to theone of electrodes 24 corresponding to the high impedance measurement.Alternatively, processor 18 may display the impedance measurements tothe user of ablation system 10, and the user may make manual adjustmentsbased at least in part on the impedance measurements.

FIG. 7 is a flow diagram illustrating exemplary operation of ablationsystem 10 adjusting one or more parameters of the delivered therapybased on one or more impedances computed for the star-configurationmodel. Ablation system 10 delivers an energy burst on electrode 24A(FIG. 1) and measures the impedance between electrode 24A and commonreturn electrode 26, i.e., R₁₃ (100). The energy is delivered toelectrode 24A without any energy being delivered to electrode 24B.Ablation system 10 also delivers an energy burst to electrode 24B andmeasures the impedance between electrode 24B and return electrode 26,i.e., R₂₃ (102). The energy is delivered to electrode 24B without anyenergy being delivered to electrode 24A. Additionally, ablation system10 delivers energy of equal amplitude voltage on both electrode 24A and24B, and measures impedance on both electrodes 24A and 24B (R₁ and R₂)(104). R₁ and R₂ may, for example, be computed using equation (5) and(6) above, in which some of the variables will be known and others maybe measured. For example, V₁ and V₂ may be known and I₁ and I₂ may bemeasured upon delivery of the energy to electrodes 24. Processor 18computes a common impedance to return electrode 26 (i.e., R₃₃) using thevariables measured during the series of energy pulses (106). Inparticular, processor 18 may compute R₃₃ using equation (9) above. Theseries of energy pulses described above may be delivered in any order toobtain the variables for computing R₃₃. Moreover, the series of energybursts may be delivered prior to initiation of the therapy or afterbeginning the therapy. In the latter case, the therapy may betemporarily disrupted.

Processor 18 determines whether it is time to re-compute R₃₃ (108).Processor may periodically compute R₃₃ to ensure proper operation ofelectrodes 24. When processor 18 determines that it is time tore-compute R₃₃, ablation system 10 delivers another energy burst onelectrode 24A (FIG. 1) and measures the impedance between electrode 24Aand common return electrode 26, i.e., R₁₃ (110). The energy is againdelivered to electrode 24A without any energy being delivered toelectrode 24B. Ablation system 10 also delivers another energy burst onelectrode 24B and measures the impedance between electrode 24B andreturn electrode 26, i.e., R₂₃ (112). The energy is again delivered toelectrode 24B without any energy being delivered to electrode 24A.Additionally, ablation system 10 delivers energy of equal amplitudevoltage on both electrode 24A and 24B, and measures impedance on bothelectrodes 24A and 24B (R₁ and R₂) (114). R₁ and R₂ may, for example, becomputed using equation (5) and (6) above, in which some of thevariables will be known and others may be measured. For example, V₁ andV₂ may be known and I₁ and I₂ may be measured upon delivery of theenergy to electrodes 24. Processor 18 computes a common impedance toreturn electrode 26 (i.e., R₃₃) using the variables measured during theseries of energy pulses (116). In particular, processor 18 may computeR₃₃ using equation (9) above.

Processor 18 computes a difference between the first computed R₃₃ andthe second computed R₃₃ value (118). Processor 18 determines whether thedifference exceeds a threshold difference (120). If the difference doesnot exceed the threshold difference, processor 18 waits until it is timeto re-compute R₃₃ again. If the difference does exceed the thresholddifference, processor 18 adjusts one or more parameters of the deliveredtherapy (122). Processor 18 may, for example, cause generator 17 toreduce the amount of energy delivered to the one of electrodes 24,relocated the electrodes, test operation of the electrodes to determineif one is broke, or the like.

FIG. 8 is a flow diagram illustrating exemplary operation of ablationsystem 10 adjusting one or more parameters of the delivered therapybased on one or more impedances computed for the star-configurationmodel. Ablation system 10 delivers an energy burst on electrode 24A(FIG. 1) and measures the impedance between electrode 24A and commonreturn electrode 26, i.e., R₁₃ (130). The energy is delivered toelectrode 24A without any energy being delivered to electrode 24B.Ablation system 10 also delivers an energy burst on electrode 24B andmeasures the impedance between electrode 24B and return electrode 26,i.e., R₂₃ (132). The energy is delivered to electrode 24B without anyenergy being delivered to electrode 24A.

Processor 18 computes a difference between R₁₃ and R₂₃ (134). Processor18 determines whether the difference exceeds a threshold difference(136). If the difference exceeds the threshold difference, processor 18adjusts one or more parameters of the delivered therapy (138). Processor18 may, for example, cause generator 17 to reduce the amount of energydelivered to the one of electrodes 24, relocated the electrodes, testoperation of the electrodes to determine if one is broke, or the like.If the difference does not exceed the threshold difference, processor 18does not adjust any parameters of the therapy (140).

Various embodiments of have been the described in this disclosure. Theseembodiments may include processors that are realized by one or moremicroprocessors, ASICs, FPGAs, or other equivalent integrated logiccircuitry. In some embodiments, a computer-readable medium may storeinstructions that cause a processor to perform the techniques describedherein. The processor may utilize several different types of storagemethods to store the computer-readable instructions for the deviceoperation and data storage. These memory and storage media types mayinclude a type of hard disk, RAM, or flash memory, e.g. CompactFlash orSmartMedia.

The preceding specific embodiments are illustrative of the practice ofthe techniques of this disclosure. It is to be understood, therefore,that other expedients known to those skilled in the art or disclosedherein may be employed without departing from the scope of the followingclaims.

1. A method for delivering therapy to a patient, the method comprising:modeling an electrical interaction between a first and second activeelectrode that deliver energy to a target tissue location of a patientand a return electrode using a star-configuration resistor model inwhich all of the resistors are coupled to each other at a common node;applying energy to one or more different combinations of the first andsecond active electrodes to compute one or more parameters of thestar-configuration resistor model; delivering a therapy to the patient;and adjusting one or more parameters of the therapy based on at leastone of the computed parameters of the star-configuration resistor model.2. The method of claim 1, wherein applying energy to the differentcombinations of the first and second active electrodes to compute theparameters comprises applying energy to the different combinations ofthe first and second active electrodes to compute the parameters of thestar-configuration resistor model prior to beginning delivery of thetherapy to the patient.
 3. The method of claim 1, wherein applyingenergy to the different combinations of the first and second activeelectrodes to compute the parameters comprises applying energy to thedifferent combinations of the first and second active electrodes tocompute the parameters of the star-configuration resistor model afterbeginning delivery of the therapy to the patient.
 4. The method of claim1, wherein applying energy to the different combinations of the firstand second electrodes to compute the parameters of thestar-configuration resistor model comprises applying energy to thedifferent combinations of the first and second electrodes to compute acommon impedance to the return electrode of the star-configurationmodel.
 5. The method of claim 4, wherein applying energy to thedifferent combinations of the first and second electrodes to compute thecommon impedance to the return electrode comprises: delivering energy tothe first active electrode without delivering energy to the secondactive electrode; measuring a first impedance during delivery of theenergy to the first electrode, wherein the first impedance is animpedance between the first active electrode and the return electrode;delivering energy to the second active electrode without deliveringenergy to the first active electrode; measuring a second impedanceduring delivery of the energy to the second electrode, wherein thesecond impedance is an impedance between the second active electrode andthe return electrode; delivering energy to both the first and the secondactive electrodes concurrently; measuring a third and fourth impedanceduring the concurrent delivery of energy to both the first and secondactive electrodes, wherein the third impedance is an impedance on thefirst electrode and the fourth impedance is an impedance on the secondelectrode; and computing the common impedance to the return electrodeusing the first, second, third and fourth impedance measurements.
 6. Themethod of claim 5, wherein computing the common impedance to the returnelectrode using the first, second, third and fourth impedancemeasurements comprises computing the common impedance to the returnelectrode in accordance with the equation:R ₃₃=((R ₁ −R ₁₃)/(R ₁ /R ₂)+(R ₂ −R ₂₃)/(R ₂ /R ₁))/2, where R₃₃ is thecommon impedance to the return electrode, R₁₃ is the first measuredimpedance, R₂₃ is the second measured impedance, R₁ is the thirdmeasured impedance and R₂ is the fourth measured impedance.
 7. Themethod of claim 4, further comprising: measuring one or more electricalparameters during delivery of the therapy; and computing adjustedimpedance values between each of the first and second active electrodesand the return electrode based on at least one of the computedparameters of the star-configuration resistor model and one or more ofthe electrical parameters measured during delivery of the therapy. 8.The method of claim 7, wherein: measuring one or more electricalparameters during delivery of the therapy comprises: measuring a firstimpedance on the first electrode; measuring a second impedance on thesecond electrode; measuring a first current through the first electrode;measuring a second current through the second electrode; and computingthe adjusted impedance values between each of the first and secondactive electrodes and the return electrode comprises: computing a firstadjusted impedance value between the first electrode and the returnelectrode using a common impedance to the return electrode of thestar-configuration model, the first measured impedance, and the firstmeasured current; and computing a second adjusted impedance valuebetween the second electrode and the return electrode using the commonimpedance to the return electrode of the star-configuration model, thesecond measured impedance, and the second measured current.
 9. Themethod of claim 8, further comprising computing the first and secondadjusted impedance values using the equations:R ₁₃ =R ₁ −R ₃₃*(I ₂ /I ₁)R ₂₃ =R ₂ −R ₃₃*(I ₁ /I ₂), where R₁₃ is the first adjusted impedancevalue, R₂₃ is the second adjusted impedance value, R₁ is the firstimpedance measurement, R₂ is the second impedance measurement, I₁ is thefirst current measurement, I₂ is the second current measurement, and R₃₃is the common impedance to the return electrode.
 10. The method of claim7, wherein adjusting one or more parameters of the therapy comprisesreducing an amount of energy delivered to the first electrode when theadjusted impedance value between the first electrode and the returnelectrode exceeds a threshold impedance.
 11. The method of claim 7,further comprising displaying the adjusted impedance values to a user.12. The method of claim 1, further comprising: applying energy to thedifferent combinations of the first and second active electrodes tocompute the parameters of the star-configuration resistor model prior tobeginning delivery of the therapy to the patient; applying energy to thedifferent combinations of the first and second active electrodes tore-compute the parameters of the star-configuration resistor model afterbeginning delivery of the therapy to the patient; comparing theparameters computed prior to beginning therapy with the parameterscomputed after beginning therapy; and adjusting one or more parametersof the therapy based on the comparison.
 13. The method of claim 1,further comprising: delivering energy to the first active electrodewithout delivering energy to the second active electrode; measuring afirst impedance during delivery of the energy to the first electrode,wherein the first impedance is an impedance between the first activeelectrode and the return electrode; delivering energy to the secondactive electrode without delivering energy to the first activeelectrode; measuring a second impedance during delivery of the energy tothe second electrode, wherein the second impedance is an impedancebetween the second active electrode and the return electrode; andcomparing the first and second measured impedances; and adjusting one ormore parameters of the therapy when the differences between the firstand second measured impedances exceeds a threshold.
 14. The method ofclaim 1, wherein adjusting one or more parameters of the therapycomprises adjusting the amount of energy delivered to at least one ofthe active electrodes.
 15. The method of claim 1, wherein adjusting oneor more parameters of the therapy comprises adjusting an amount of fluiddelivered to the target tissue location.
 16. The method of claim 1,wherein adjusting one or more parameters of the therapy comprisesadjusting a location of one of the active electrodes.
 17. A therapydelivery device comprising: a first active electrode that deliversenergy to a target tissue location of a patient; a second activeelectrode that delivers energy to the target tissue location of thepatient; a return electrode that operates as a destination for theenergy delivered via the first and second active electrodes; a generatorthat delivers therapy energy to the patient via the first and secondactive electrodes; and a processor that models an electrical interactionbetween the first and second active electrode and the return electrodeusing a star-configuration resistor model in which all of the resistorsare coupled to each other at a common node and adjusts one or moreparameters of a therapy based on at least one computed parameter of thestar-configuration resistor model.
 18. The device of claim 17, wherein:the generator applies energy to one or more different combinations ofthe first and second active electrodes, and the processor computes oneor more parameters of the star-configuration resistor model based onmeasurements made during application of energy to the differentcombinations of the active electrodes,
 19. The device of claim 18,wherein the generator applies energy to the different combinations ofthe first and second active electrodes and the processor computes theparameters of the star-configuration resistor model prior to beginningdelivery of the therapy to the patient.
 20. The device of claim 18,wherein the generator applies energy to the different combinations ofthe first and second active electrodes and the processor computes theparameters of the star-configuration resistor model after beginningdelivery of the therapy to the patient.
 21. The device of claim 17,wherein the processor computes a common impedance to the returnelectrode of the star-configuration model.
 22. The device of claim 21,wherein: the generator delivers energy to the first active electrodewithout delivering energy to the second active electrode; the measuringcircuit measures a first impedance during delivery of the energy to thefirst electrode, wherein the first impedance is an impedance between thefirst active electrode and the return electrode; the generator deliversenergy to the second active electrode without delivering energy to thefirst active electrode; the measuring circuit measures a secondimpedance during delivery of the energy to the second electrode, whereinthe second impedance is an impedance between the second active electrodeand the return electrode; the generator delivers energy to both thefirst and the second active electrodes concurrently; the measuringcircuit measures a third and fourth impedance during the concurrentdelivery of energy to both the first and second active electrodes,wherein the third impedance is an impedance on the first electrode andthe fourth impedance is an impedance on the second electrode; and theprocessor computes the common impedance to the return electrode usingthe first, second, third and fourth impedance measurements.
 23. Thedevice of claim 22, wherein the processor computes the common impedanceto the return electrode using the first, second, third and fourthimpedance measurements comprises computing the common impedance to thereturn electrode in accordance with the equation:R ₃₃=((R ₁ −R ₁₃)/(R ₁ /R ₂)+(R ₂ −R ₂₃)/(R ₂ /R ₁))/2, where R₃₃ is thecommon impedance to the return electrode, R₁₃ is the first measuredimpedance, R₂₃ is the second measured impedance, R₁ is the thirdmeasured impedance and R₂ is the fourth measured impedance.
 24. Thedevice of claim 17, further comprising: a measuring circuit thatmeasures one or more electrical parameters during delivery of thetherapy; wherein the processor computes adjusted impedance valuesbetween each of the first and second active electrodes and the returnelectrode based on the at least one computed parameter of thestar-configuration resistor model and one or more of the electricalparameters measured during delivery of the therapy.
 25. The device ofclaim 24, wherein: the measuring circuit measures a first impedance onthe first electrode, a second impedance on the second electrode, a firstcurrent through the first electrode, and a second current through thesecond electrode; and the processor computes a first adjusted impedancevalue between the first electrode and the return electrode using acommon impedance to the return electrode, the first measured impedance,and the first measured current and computes a second adjusted impedancevalue between the second electrode and the return electrode using thecommon impedance to the return electrode, the second measured impedance,and the second measured current.
 26. The device of claim 25, wherein theprocessor computes the first and second adjusted impedance values usingthe equations:R ₁₃ =R ₁ −R ₃₃*(I ₂ /I ₁)R ₂₃ =R ₂ −R ₃₃*(I ₁ /I ₂), where R₁₃ is the first adjusted impedancevalue, R₂₃ is the second adjusted impedance value, R₁ is the firstimpedance measurement, R₂ is the second impedance measurement, I₁ is thefirst current measurement, I₂ is the second current measurement, and R₃₃is the common impedance to the return electrode.
 27. The device of claim25, wherein the processor causes the generator to reduce the amount ofenergy delivered to the first electrode when the adjusted impedancevalue between the first electrode and the return electrode exceeds athreshold impedance.
 28. The device of claim 25, further comprising adisplay that displays the adjusted impedance values to a user.
 29. Thedevice of claim 17, wherein the processor computes the at least oneparameter of the star-configuration resistor model prior to beginningdelivery of the therapy to the patient, computes the at least oneparameter of the star-configuration resistor model after beginningdelivery of the therapy to the patient, compares the parameter computedprior to beginning therapy with the parameter computed after beginningtherapy, and adjusts one or more parameters of the therapy based on thecomparison.
 30. The device of claim 17, wherein: the generator deliversenergy to the first active electrode without delivering energy to thesecond active electrode; the measuring circuit measures a firstimpedance during delivery of the energy to the first electrode, whereinthe first impedance is an impedance between the first active electrodeand the return electrode; the generator delivers energy to the secondactive electrode without delivering energy to the first activeelectrode; the measuring circuit measures a second impedance duringdelivery of the energy to the second electrode, wherein the secondimpedance is an impedance between the second active electrode and thereturn electrode; and the processor compares the first and secondmeasured impedances and adjusts one or more parameters of the therapywhen the differences between the first and second measured impedancesexceeds a threshold.
 31. The device of claim 17, wherein the processoradjusts the amount of energy delivered to at least one of the activeelectrodes.
 32. The device of claim 17, wherein the processor adjusts anamount of fluid delivered to the target tissue location.
 33. Acomputer-readable medium comprising instructions that cause a processorto: model an electrical interaction between a first and second activeelectrode that deliver energy to a target tissue location of a patientand a return electrode using a star-configuration resistor model inwhich all of the resistors are coupled to each other at a common node;apply energy to one or more different combinations of the first andsecond active electrodes to compute one or more parameters of thestar-configuration resistor model; deliver a therapy to the patient; andadjust one or more parameters of the therapy based on at least one ofthe computed parameters of the star-configuration resistor model. 34.The computer-readable medium of claim 33, wherein the instructions thatcause the processor to apply energy to one or more differentcombinations of the first and second active electrodes to compute theparameters comprise instructions to cause the processor to apply energyto the different combinations of the first and second active electrodesto compute the parameters of the star-configuration resistor model priorto beginning delivery of the therapy to the patient.
 35. Thecomputer-readable medium of claim 33, wherein the instructions thatcause the processor to apply energy to one or more differentcombinations of the first and second active electrodes to compute theparameters comprise instructions to cause the processor to apply energyto the different combinations of the first and second active electrodesto compute the parameters of the star-configuration resistor model afterbeginning delivery of the therapy to the patient.
 36. Thecomputer-readable medium of claim 33, wherein the instructions thatcause the processor to apply energy to one or more differentcombinations of the first and second active electrodes to compute theparameters comprise instructions to cause the processor to apply energyto the different combinations of the first and second active electrodesto compute a common impedance to the return electrode of thestar-configuration model.
 37. The computer-readable medium of claim 36,wherein the instructions that cause the processor to compute the commonimpedance to the return electrode comprise instructions that cause theprocessor to: deliver energy to the first active electrode withoutdelivering energy to the second active electrode; measure a firstimpedance during delivery of the energy to the first electrode, whereinthe first impedance is an impedance between the first active electrodeand the return electrode; deliver energy to the second active electrodewithout delivering energy to the first active electrode; measure asecond impedance during delivery of the energy to the second electrode,wherein the second impedance is an impedance between the second activeelectrode and the return electrode; deliver energy to both the first andthe second active electrodes concurrently; measure a third and fourthimpedance during the concurrent delivery of energy to both the first andsecond active electrodes, wherein the third impedance is an impedance onthe first electrode and the fourth impedance is an impedance on thesecond electrode; compute the common impedance to the return electrodeusing the first, second, third and fourth impedance measurements. 38.The computer-readable medium of claim 37, wherein the instructions thatcause the processor to compute the common impedance to the returnelectrode using the first, second, third and fourth impedancemeasurements comprises instructions that cause the processor to computethe common impedance to the return electrode in accordance with theequation:R ₃₃=((R ₁ −R ₁₃)/(R ₁ /R ₂)+(R ₂ −R ₂₃)/(R ₂ /R ₁))/2, where R₃₃ is thecommon impedance to the return electrode, R₁₃ is the first measuredimpedance, R₂₃ is the second measured impedance, R₁ is the thirdmeasured impedance and R₂ is the fourth measured impedance.
 39. Thecomputer-readable medium of claim 33, further comprising instructionsthat cause the processor to: measure one or more electrical parametersduring delivery of the therapy; and compute adjusted impedance valuesbetween each of the first and second active electrodes and the returnelectrode based on at least one of the computed parameters of thestar-configuration resistor model and one or more of the electricalparameters measured during delivery of the therapy.
 40. Thecomputer-readable medium of claim 39, wherein: the instructions thatcause the processor to measure one or more electrical parameters duringdelivery of the therapy comprise instructions that cause the processorto measure one or more of: a first impedance on the first electrode; asecond impedance on the second electrode; a first current through thefirst electrode; a second current through the second electrode; and theinstructions that cause the processor to compute the adjusted impedancevalues between each of the first and second active electrodes and thereturn electrode comprise instructions that cause the processor to:compute a first adjusted impedance value between the first electrode andthe return electrode using the common impedance, the first measuredimpedance, and the first measured current; and compute a second adjustedimpedance value between the second electrode and the return electrodeusing the common impedance, the second measured impedance, and thesecond measured current.
 41. The computer-readable medium of claim 40,further comprising instructions that cause the processor to compute thefirst and second adjusted impedance values using the equations:R ₁₃ =R ₁ −R ₃₃*(I ₂ /I ₁)R ₂₃ =R ₂ −R ₃₃*(I ₁ /I ₂), where R₁₃ is the first adjusted impedancevalue, R₂₃ is the second adjusted impedance value, R₁ is the firstimpedance measurement, R₂ is the second impedance measurement, I₁ is thefirst current measurement, I₂ is the second current measurement, and R₃₃is the common impedance to the return electrode.
 42. Thecomputer-readable medium of claim 33, further comprising instructionsthat cause the processor to display the adjusted impedance values to auser.
 43. The computer-readable medium of claim 33, further comprisinginstructions that cause the processor to: apply energy to the differentcombinations of the first and second active electrodes to compute theparameters of the star-configuration resistor model prior to beginningdelivery of the therapy to the patient; apply energy to the differentcombinations of the first and second active electrodes to re-compute theparameters of the star-configuration resistor model after beginningdelivery of the therapy to the patient; compare the parameters computedprior to beginning therapy with the parameters computed after beginningtherapy; and adjust one or more parameters of the therapy based on thecomparison.
 44. The computer-readable medium of claim 33, furthercomprising instructions that cause the processor to: deliver energy tothe first active electrode without delivering energy to the secondactive electrode; measure a first impedance during delivery of theenergy to the first electrode, wherein the first impedance is animpedance between the first active electrode and the return electrode;deliver energy to the second active electrode without delivering energyto the first active electrode; measure a second impedance duringdelivery of the energy to the second electrode, wherein the secondimpedance is an impedance between the second active electrode and thereturn electrode; and compare the first and second measured impedances;and adjust one or more parameters of the therapy when the differencesbetween the first and second measured impedances exceeds a threshold.45. The computer-readable medium of claim 33, wherein instructions thatcause the processor to adjust the parameters comprise instructions thatcause the processor to adjust an amount of energy delivered to at leastone of the active electrodes.
 46. The computer-readable medium of claim33, wherein instructions that cause the processor to adjust theparameters comprise instructions that cause the processor to adjust anamount of fluid delivered to the target tissue location.